Abstract: The p53 transcription factor is a key regulator of the DNA damage response, genome stability and tumor suppression. Because of its potent apoptotic activity, p53 expression must be tightly regulated through multiple mechanisms to ensure a robust and precise response to cellular stress signals. Our laboratory previously identified the REDD1 gene as a direct p53 transcriptional target induced following DNA damage. REDD1 is known to function as an essential suppressor of mTORC1 kinase activity in response to cellular stress including hypoxia and energy stress. Loss of REDD1 cooperates in p53-deficient cells in vitro to induce tumorigenesis, and REDD1 silencing is observed in a subset of human tumors. Nevertheless, the specific contribution of REDD1 to p53-mediated apoptotic function had yet to be clearly defined. I provide evidence that REDD1-null cells and tissues exhibit increased sensitivity to DNA damage in vitro and in vivo. This increased sensitivity to apoptosis is attributable to an increase in p53 protein levels in REDD1-null cells, caused by enhanced p53 translation. Taken together, these studies reveal a complex pattern of genetic interactions between REDD1 and p53 in apoptosis and tumorigenesis. Based on these data, I hypothesize that REDD1 may function analogously to tumor suppressors such as BRCA1 and Rb, whose loss in primary cells triggers p53 activation, but which cooperates in p53 deficient cells to promote tumorigenesis in vivo. I propose three aims in order to uncover the mechanism of this novel p53-REDD1 feedback pathway and to define the genetic interaction between REDD1 and p53 in tumorigenesis in vivo. First, I will determine the underlying mechanism of increased p53 translation in REDD1-null cells, examining both mTORC1-dependent and independent pathways for translational regulation. Second, I will test whether increased p53 observed in REDD1-null cells may serve as a barrier to cellular transformation and tumorigenesis, by examining the ability of REDD1-null cells and mice to undergo oncogene-induced anchorage-independent growth and chemically-induced tumorigenesis. Finally, I will cross REDD1-null and p53-null mice to test the hypotheses that loss of p53 will abrogate REDD1-dependent DNA damage sensitivity, and that loss of REDD1 will enhance p53-dependent tumorigenesis. Together, my findings are likely to provide a better understanding of the link between cellular stress responses and tumorigenesis.